This invention relates to load control in an electrical distribution system; and, more particularly, to a method of performing load control using temporal measurements of energy usage by individual pieces of equipment connected to the system.
Electrical utilities must continually manage their capacity to ensure that the amount of electricity generated by the utility, or purchased from other utilities, is sufficient to meet the load demand placed on the system by their customers. Utilities generally have two options for meeting demands on the system during periods of peak energy demand (loading). These include either bringing additional generating capacity on-line to satisfy the increased demand; or, if properly equipped, shedding load across their customer base to reduce overall demand on the system.
Demand response thus refers to the reduction of a customer's energy usage at times of peak demand. It is done for a variety of reasons including system reliability (the avoidance of “blackouts” or “brownouts”), market conditions and pricing (preventing the utility from having to buy additional energy on the open-market at premium prices), and supporting infrastructure optimization or deferral. Demand response programs include dynamic pricing/tariffs, price-responsive demand bidding, contractually obligated and voluntary curtailment of energy usage, and direct load control/cycling.
When reducing demand, it is desirable to equitably distribute the necessary load shedding across the customer base. This is especially true where participation in load control programs is voluntary. In this regard, a number of methods have been proposed to manage load control fairly across a wide range of customers and their individual needs. These methods make use of demand and/or rate of demand as measured at a customer's site. This amount of “dispatchable” load, i.e., usage that can be shed at a given time, is calculated from these measurements and then used to formulate set points and/or generate control signals which directly affect the shedding of load.
Directly measuring energy usage, however, has a number of drawbacks. For example, to measure demand at a site, either a demand type metering device must be used, or a similar demand metering capability must be present in a load control device employed at that location. While some utilities may only employ demand measuring capability for a short time, i.e., until robust models are developed, even the short term deployment of a measuring capability may not only be cost prohibitive, but also require additional levels of system management. Also, measuring usage at the premise level does not provide an indication of usage patterns at the equipment level. The result is that control signals meant to control individual loads are based on global measurements that have been taken and which are applied equally across all controlled loads. Generally speaking, utilities are primarily concerned with usage on an aggregate level, and individual equipment level data is not considered. However, the ability to refine load control to a higher degree of resolution can produce greater accuracies and better performance in load control strategies a utility may potentially employ.
Accordingly, it is desirable to profile equipment energy usage in a way that does not incur the extra cost and/or complexity of load demand metering while not reducing the effectiveness of control. Furthermore, it is desirable to provide more than just profile usage at a customer's premise, but to also profile the usage for each piece of equipment individually. This is done so that the demand of each controlled device is factored in, so to promote a sophisticated control strategy which reduces the perceived impact by a customer on them during a load control event.
Another shortcoming of current load control methods is that, however calculated, the load control unit employed imposes an artificial duty cycle on operation of the controlled equipment. Even in the event that the control signal is computed from a usage profile of the equipment, the signal is imposed without regard as to whether the equipment is actually operating. That is, these methods have no mechanism which ensures that a piece of equipment (the “load”) actually needs power at a time when a load control event is occurring. For example, if a piece of equipment is “off” and not needed or required to be “on”, then it has no need either for power, or for a reduction in power, at that time. A method which attempts to determine or set the power requirement for equipment during a load control event is therefore imposing an artificial burden on the system.
In effect, in these prior load control systems, the control signal only sets an upper limit of usage on the equipment. This then implies that the equipment may not “call” for power, when it is allowed to call, and will not run, even if it could. This impacts the customer's perception of a load control event by giving the appearance of a higher amount of load being shed than actually is, in response to operation of the load control system. It is therefore desirable to improve upon load control methods where only an aggregate level of control is considered, by maximizing load control at the equipment level. It is further desirable to be able to synchronize those periods during which equipment is allowed to run with those periods when the equipment is actually calling for power. Certain loads, those driven by thermostats, for example, will “self-synchronize” with the load control as they continually endeavor to reach certain set points. However, many loads are driven only on a time basis, or by some other mechanism, that does not “self-synchronize” with load control signals.
When load control is performed, the utility prefers to have some form of feedback as to the effectiveness of the load control commands. Most of the existing ways of providing this feedback involve either using metering data obtained from the premise before, during, and after the load control event to verify that load control was performed; or, to use counters that indicate how many times power to the device was cycled “on” and “off”. The former method requires that data be monitored and generally requires additional equipment in the load control system to make the measurements. The latter method does not give a measure of effectiveness straight away. Rather, calculations are performed to determine if it can be inferred that a piece of equipment was cycled “off” ahead of when it otherwise would be, thus indicating that load was shed. Even load control methods employing sophisticated models have an operational disadvantage because of the number of computations needed to provide meaningful feedback to a utility.
In addition to the complexities inherent in the modeling, the resulting models do not necessarily reflect the fact that a given consumer might suffer from, for example, air-conditioner under-capacity. What might appear from the utility's perspective to be a very good load to shed because it runs all of the time, might be from the customer's perspective a very bad load to shed because it needs to run all of the time. The customer may, for example, experience undue discomfort when the program is executed, and opt out of the program, which is not a favorable result for the utility. To avoid this situation, some utilities study their customer's appliances and usage habits, and refine their models accordingly. Some load control systems employ counters for this purpose and are thereby able to monitor equipment cycling in such a way as to keep their customers comfortable. It will be appreciated that this requires a great deal of planning and forethought. The present invention attempts to keep individual customers comfortable without requiring sophisticated modeling by the utility. To accomplish this, it is desirable to have a load control unit (LCU) both perform the load control and to directly report its performance back to the utility. Doing so reduces the need for additional system resources, and reduces the number of calculations needed to be performed by the utility.
A final identified item is an alternative to traditional load control methods of dealing with additional loading on the electrical network when the load control event is over. At the time that equipment is allowed to come back online, all of it may simultaneously switch “on” and significantly increase the level of demand on the system as the equipment attempts to heat water, lower room temperatures, etc. This increased demand is counter-productive to the original need for load control. Indeed, if the need for load control was triggered by a reliability concern, then this increase in demand may retrigger the event and load control will start over again. One method of managing this situation in current load control methods is to employ time diversification by which controlled loads are randomly brought back on line in order to spread any in-rush current that is generated over a longer time period, thus reducing peak loads. An alternative posed by this invention is to gradually ease all of the equipment back online and so slowly reduce the amount of load being shed. This method does not require time diversification. Instead, all loads are treated equally with respect to timing while the magnitude of load shed is gradually reduced over a period of time until all of the loads are fully brought out of control. In accordance with the invention, this is readily accomplished since the loads are controlled at the equipment level, and this provides a natural diversification.
In the discussion that follows, it is important for those skilled in the art to understand that the terms “usage”, “energy usage”, “profile”, “usage profile” and related terms, as used herein, do not refer to the amount of energy (amps or watts) a piece of equipment is consuming at any particular time. Rather, what is referred to is the amount of time the piece of equipment is running during a defined interval (half-hour, hour, day) regardless of the level of consumption. It will also be understood that certain equipment (e.g., an air conditioner) may be continuously drawing current; albeit a relatively low amount of current as compared to when the equipment is performing its actual function (i.e., cooling a space). For purposes of operation of the invention, a piece of equipment is considered to be “on” or running, whenever its level of energy consumption exceeds a predetermined threshold level specific to that piece or type of equipment, regardless of the various amounts of energy thereafter consumed at different times when the equipment is running.